1. Field of the Invention
The present invention relates to a magneto-resistive effect element aimed at improving output stability, and a magnetic disk device using the same.
2. Description of the Related Art
A magneto-resistive effect element is an element making use of a physical phenomenon such that magneto-resistivity largely varies depending on a relative angle of magnetization between two magnetic layers. The magneto-resistive effect element can be exemplified by a giant magneto-resistive effect (GMR) element and a tunnel-type magneto-resistive effect (TMR) element, for example. The magneto-resistive effect element applied to a reading head of a magnetic head generally adopts a spin-valve-type structure. In the spin-valve-type structure, magnetization in one magnetic layer is pinned, and magnetization in the other magnetic layer is allowed to freely move in response to the external magnetic field. In order to stably operate the magneto-resistive effect element applied to the magnetic head, it is necessary to thoroughly pin the one magnetization. For pinning of the magnetization in the magnetic layer, a generally adopted method is such as producing an exchange-coupled magnetic field by stacking an anti-ferromagnetic layer and a ferromagnetic layer. Enhancement of the exchange-coupled magnetic field is considered as making large contributions in improvement in stability, and also in output, of the magneto-resistive effect element.
Structures of the magneto-resistive effect element are roughly classified into the CIP (current-in-plane) structure and the CPP (current-perpendicular-to-plane) structure. A sense current in the CIP structure flows through a film composing the magneto-resistive effect element in the in-plane direction, and a sense current in the CPP structure flows in the direction normal to a film composing the magneto-resistive effect element.
In the magneto-resistive effect element having the CIP structure, as shown in FIG. 7, an alumina film 102 is formed on a substrate 101, and thereon a NiFe plated film is formed as a lower shield layer 104. Further on the lower shield layer 104, an amorphous alumina film 103, a GMR film 105, a hard layer 107, an electrode 109, an alumina film 106 and an upper shield layer 108 are formed. A NiFe plated film is formed as the upper shield layer 108.
In the GMR film 105, an underlying film, an anti-ferromagnetic film, a ferromagnetic pinned layer, a non-magnetic intermediate layer, a ferromagnetic free layer and a cap layer are stacked on the alumina film 103. Formation of the underlying film on the alumina film 103 in an amorphous state results in a large grain size of crystal grains in the underlying film, and results in a large grain size of crystal grains also in the anti-ferromagnetic film. As a consequence, magnetization in the anti-ferromagnetic film is stabilized, and the exchange-coupled magnetic field is intensified.
As has been described in the above, the magneto-resistive effect element having the CIP structure can intensify the exchange-coupled magnetic field, by using the amorphous alumina film 103, and consequently by enlarging the crystal grains of the anti-ferromagnetic film formed thereon.
As a trend towards higher recording densities in magnetic recording media advances, there are growing demands for further down-sizing and larger sensitivity. In this situation, the magneto-resistive effect element of the CPP-type is more effective than the magneto-resistive effect element of the CIP-type. The CPP-type, magneto-resistive effect element yields a larger output as the element size decreases. Also a TMR film making use of a tunnel-type magneto-resistive effect is often used, in place of the GMR film.
In the magneto-resistive effect element having the CPP structure, as shown in FIG. 8, an alumina film 202 is formed on a substrate 201, and thereon a NiFe plated film is formed as a lower shield layer 204. Further on the lower shield layer 204, a GMR film 205, an alumina film 206, a hard layer 207 and an upper shield layer 208 are formed. A NiFe plated film is formed as the upper shield layer 208.
In the GMR film 205, an underlying film, an anti-ferromagnetic film, a ferromagnetic pinned layer, a non-magnetic intermediate layer, a ferromagnetic free layer and a cap layer are stacked on the lower shield layer 204. A TMR film may sometimes be used in place of the GMR film 205. In this case, a tunnel insulating film is used in place of the non-magnetic intermediate layer.
In the magneto-resistive effect element of the CPP type, the alumina film, which is an insulating film, cannot be provided between the GMR film 205 and the lower shield layer 204, because it is necessary to allow current to flow between the upper shield layer 208 and the lower shield layer 204. As the lower shield layer 204, a soft magnetic film mainly composed of NiFe or the like is formed by plating. The soft magnetic film formed by plating has a crystal grain size of several tens nanometers or around, and also the anti-ferromagnetic film epitaxially grown thereon has a crystal grain size of several tens nanometers or around. In other words, the magneto-resistive effect element of the CPP type cannot enlarge the crystal grain size of the anti-ferromagnetic film as compared with that in the CIP type. Therefore, the exchange-coupled magnetic field cannot be intensified to a satisfactory degree, and the problem results in only an insufficient output of the magneto-resistive effect element, and/or degraded stability of the output.
On the other hand, the intensity of the exchange-coupled magnetic field reportedly depends on orientation of the crystal grains in the anti-ferromagnetic film (Masakiyo Tsunoda et al., J. Appl. Phys., 87, 4375 (2000)). However, in the conventional magneto-resistive effect elements of the CPP structure, the lower shield layer 204 is composed of gathering of fine crystal grains showing almost no crystal orientation. It is therefore impossible to control the crystal orientation of the anti-ferromagnetic film epitaxially grown thereon.
A related art is disclosed in Japanese Patent No. 3295013.